Page:A Treatise on Geology, volume 2.djvu/259

 CHAP. IX.

From the point where the earthquake originates, two sets of waves proceed in the solid crust of the earth, viz., the wave of elastic compression, propagated in every direction with a velocity proportioned to the elasticity and density of the parts of the earth-crust in its path. In different sorts of rock the velocity will not be the same: it will be greatest in the solid, and least in the loose aggregations of matter. Another set of waves is that of sound. And, if the origin of the earthquake be under the sea, a water wave of translation will be generated in the sea, of much less velocity than that in the earth. Sound waves will be communicated to the water and to the air; but of these we need not say much. If the earthquake originate under the land, and be propagated under the sea, it will reach the extreme border of the sea, and raise the shore so as to force the water to appear to retire, and afterwards to return and flow higher than before, a phenomenon distinctly observed. Supposing the first shock to have happened under the sea, and all the waves to be noticed on the extreme edge of the water, we should have,

1 . The earth waves of shock and sound together, or nearly so.

2. The forced sea wave lost upon the beach.

3. The sound wave through the sea.

4. Sound waves (possibly) through the air.

5. The great water-wave, which has been found so destructive.

According to Mr. Mallet, the velocities to be expected in the sound-wave would be 4700 feet per second in water, 1140 feet in air; and, judging from the elasticity, in lias 3640; in coal measure sandstones 5248; in oolite 5723; in primary limestone 6696; in carboniferous limestone 7075; and in hard slate 1 2,757; and in granite and igneous rocks still higher rates. Perhaps the speed of the great earth-wave may be nearly the same; but the masses of rock in the earth are so much interrupted by joints, by unequal condensation, varying